Did you contact them personally or something? If so that would be an important detail to mention.
Anyway nobody really "owns" file extensions - given how long PCs have been around and how many pieces of software have been written, it's inevitable that some file extensions are going to be used differently by more than one program. It's also trivial for a PC user to change the extension of a file without changing the actual content of the file. I guarantee that .mod is used by some applications as something other than music, and probably .xm and .it as well (maybe not .s3m though).